This invention relates generally to the manufacture and assembly of color cathode ray tubes (CRTs) and is particularly directed to the processing of shadow masks used in color CRTs.
The shadow mask is a part of the CRT front assembly and is located closely adjacent to the CRT's faceplate. The CRT front assembly primarily comprises the faceplate with its screen consisting of a black matrix and deposits of light-emitting phosphors, the shadow mask and support means for the shadow mask. In the past, domed shadow masks have been cleaned prior to installation in a color CRT using an ultrasonic bath containing de-ionized water and in some cases a dispersing agent or a surfactant. The present invention may be used in cleaning the standard, domed shadow mask or the recently developed flat tension masks (FTMs) used in color CRTs having a flat glass faceplate. As used herein, the term "FTM shadow mask", or "mask" means an apertured metallic foil which may, by way of example, be approximately 0.001 inch thick, or less. The following description is directed toward use of the present invention with FTM shadow masks, it being understood that this discussion is equally applicable to standard domed shadow masks.
The FTM shadow mask must be securely supported and maintained in high tension a predetermined distance from the inner surface of the CRT faceplate; this distance is known as the "Q-distance". Attachment of the FTM shadow mask may be by various means, typically by welding. As is well known in the art, the FTM shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of the three electron beams lands only on its assigned phosphor deposits. The FTM shadow mask may either be "new", i.e., its first use the manufacturing process, or "used", i.e., recovered from a rejected front assembly for re-installation in another front assembly. FTM shadow mask recovery for re-installation is justified by the high cost of these color CRT components. It is these recovered FTM shadow masks which present particular problems in terms of contaminant removal and disposal.
During CRT assembly, the FTM shadow mask is securely attached with its support frame to the inner surface of the CRT's flat glass faceplate. Referring to FIG. 1, there is shown a plan view of a first side of a prior art factory fixture frame 13 for use in maintaining an FTM shadow mask in a tightly stretched condition prior to installation in a color CRT. The factory fixture frame 13 is disclosed and claimed in the above cross-referenced patent, the disclosure of which is incorporated herein by reference. The factory fixture frame 13 provides for high precision in the registration of a flat in-process shadow mask with a faceplate during manufacture. The reusable factory fixture frame 13 includes a first side and comprises a generally rectangular frame means and quick-release mechanical mask-retaining means for temporarily and removably supporting an in-process shadow mask 14 in tension. Frame 13 is indicated as supporting shadow mask 14 in tension by means of mechanical mask-retaining means 88. Factory fixture frame 13 provides for the cementless and weldless quick-retention of in-process shadow mask 14 out of the plane of the mask. The factory fixture frame 13 includes handles 90A, 90B and 90C for convenience in handling during manufacture as the factory fixture frame is inserted in and removed from a mask tensing-clamping machine (not shown in the figure for simplicity). When the in-process mask 14 is fully expanded by the heat of upper and lower platens (also not shown), the mask is clamped, and the platens are withdrawn. The mask tenses as it cools, and is held in tension by the clamping means 88 which are a component of the factory fixture frame 13. Groove means 132a, 132b and 132c provide for proper alignment and registration of the FTM shadow mask 14 as the factory fixture frame 13 and shadow mask are lowered into registration with a lighthouse (not shown) for exposing the screening surface of an in-process faceplate to radiation from a light source within the lighthouse. Other functions performed by the factory fixture frame 13 and additional structural details thereof are described in the aforementioned cross-referenced patent. During processing and attachment of the FTM shadow mask, various contaminants are produced which may inhibit CRT assembly and may even degrade CRT performance following manufacture.
For example, during the processing of the panel, the FTM shadow mask may inadvertently come in contact with various chemical materials. Most often, these materials may become lodged in the precisely-etched apertures of the shadow mask. When this happens, the photographic process involved in the manufacture of the dark surround (also called "grille" or "black matrix") is inhibited to such a localized extent that the screen on the faceplate must be rejected. In addition, the various chemical cleaning agents may form a residue on the periphery of the FTM shadow mask. As little as 0.001" of slurry residue on the mask can change the Q-distance upon installation on the mask-supporting rails enough to introduce electron beam landing errors. Deposits of slurry residue on the periphery of the FTM shadow mask may also prevent good bonding, such as via a weldment, between the mask and its support structure.
Another case of this contamination is where the aforementioned chemical material is on the border of the shadow mask array. In the case of conventional CRT product, failure to remove this material can provide a source of particles which can move to the CRT gun and inhibit its operation, or can move onto the array of the shadow mask and interfere with the electron beam by causing a plug or a charged particle. These are both cause for rejection of the product. In the case of FTM shadow masks, this material can present a barrier between the shadow mask and its support structure and prevent satisfactory welding of these parts--a necessary operation in the fabrication of this product.
There are a variety of sources of contamination, including packaging materials from the shadow mask manufacturer as well as various solutions and slurries used in the process of manufacturing CRTs. In addition, workers in the manufacturing process generate contamination from saliva, skin and hair which require removal from the shadow mask before processing.
Prior attempts to clean shadow masks in automated equipment have generally involved using ultrasonic transducers in conjunction with water to which non-reactive chemicals have been added.
The present invention addresses the aforementioned limitations of the prior art and provides an improved shadow mask washing system for cleaning in-process shadow masks by directing ultrasonic pressure waves on the shadow mask, followed by rinsing, and then drying the shadow mask. The ultrasonic bath stage makes use of an improved cleaning solution having a chemically active component and allows for removal of the contaminants which inhibit shadow mask installation and degrade its performance. This invention also prolongs the life of washing system filters, requiring less frequent replacement of the filter elements.